The main purpose of this study is to evaluate a combined ozone and microfiltration (MF) membrane process as pre-treatment of a seawater reverse osmosis (SWRO) system. During the filtration of seawater (SW) with the MF membranes, membrane fouling on both hydrophilic (PHI) and hydrophobic (PHO) MF membranes (PVDF, 0.22 mu m pore size) was investigated by measuring the variation of flux. A significant decline of flux was observed in both PHI and PHO membranes, showing much higher fouling in the PHO membrane than the PHI membrane, with and without pre-filtration. Ozonation of the SW was performed to investigate the effect of ozone on fouling of the MF membrane. Reduction of membrane fouling by ozone occurred in only the PHO membrane. In the case of the PHI membrane, pre-ozonation of the SW led to more severe fouling during filtration than was the case for non-ozonated SW.